Field of the invention
The present invention concerns the application of discrete delays to a signal, in particular to a microwave electrical signal possibly modulated by a signal conveying information. It applies in particular to a phased array antenna system.
A known device for applying discrete delays comprises:
an optical emitter the intensity of which can be modulated and which is adapted to receive a signal to be delayed constituting an input signal and to supply an optical carrier wave modulated by said input signal,
an optical delay line in the form of an optical fiber adapted to receive said wave and to guide it and apply a delay to it,
a receiver adapted to receive said wave at the output of said optical delay line and to supply an output signal representative of the modulation it carries, and
means for controlling the delay applied to said wave.
The result is usually referred to as a "variable delay line". The need for opto-electronic devices of this kind in microwave applications has often been pointed out, including in various prior art documents listed at the end of this description, in particular the first of those documents.
The propagation time of the carrier wave in a line of this kind can be varied in discrete steps by switching between fibers of different lengths.
Electro-optical switches are used for changing the optical delay line under electrical control. If a single switching stage is used, the number of optical delay lines of the device is equal to the number of different delays required (see document 2). This number is reduced if a number of switching stages are connected in series (see document 3). It remains large if the number of delays is large, however.
One object of the present invention is to reduce the number of optical delay lines needed to apply a given number of different delays to an electrical signal.